Various methods have been used to present agriculturally effective active ingredients (AIs) in a solid form. The most common methods include spray drying and granulations.
Spray drying is performed typically by passing an aqueous slurry of ground AI and a binder material (usually a number of materials based on alkylnaphthylene or alkylformaldehyde condensate, calcium silicate, kaolinite, diatomaceous clays) through a nozzle into a tower. The droplets are dried at a temperature of about 150.degree. C. As the water is vaporized, the slurry droplets form the particulate product and are collected. Despite the high temperature drying, contact between the slurry water and the amount of residual adsorbed water in the binder can degrade many agriculturally useful Ais during storage. Spray drying is can be performed with water insoluble Ais if an emulsion is first formed.
Granulation can be performed by spraying an AI onto a ground carrier. Other granulation processes include low pressure extrusion, briquetting, and pelletizing. The particle size from these processes is generally about 2000-4000 .mu.m carrying 20 wt % or less of the AI.
Encapsulation is an alternative to spray drying and granulation that can provide a number of advantages for various active ingredients (Ais). In general, encapsulation in a binder can render Ais easy to handle, reduce or eliminate exposure concerns compared to the pure AI, as well as provide a measure of control over the rate, timing, and duration of AI release depending on the encapsulating material and the AI.
A product that is successfully encapsulated must consider a number of differing and often competing needs. For example, encapsulated baits must provide a structure and chemistry that considers the target insect or animal behavior, the application method, and any handling and environmental concerns for the AI. Each AI and intended can and often do result in differing encapsulation forms and formulations.
Polyethylene glycol (PEG) has generated some degree of interest in the art. PEG is a water soluble film-forming polymer that is commercially available in a wide variety of molecular weight solids. PEG has been used in a number of ways for the encapsulation of various Ais.
Pasin U.S. Pat. No. 3,664,963 describes the use of a PEG bath to remove solvent from particles containing an active ingredient, a solvent-soluble shell forming material, and a shell solvent that are sprayed into the PEG bath. As solvent is desorbed into the PEG, the shell forming material surrounds and encapsulates the active ingredient.
Snipes U.S. Pat. No. 4,629,621 and its continuation-in-part Snipes et al. U.S. Pat. No. 4,806,337 describe a cylindrical pill made by injection molding and having a controllable rate of release. In the pill is an active ingredient dispersed in a matrix containing 5-95% PEG and 5-95% of a water insoluble, amphophilic erosion rate modifier. The erosion modifier either slows the dissolution rate of the PEG to prolong the delivery period of the active ingredient or increases the dissolution rate for a faster release based on pH or moisture content of the surrounding system. The active ingredient is described generally as an ecological agent of an unspecified loading level, or "up to 70% by weight" of a pharmaceutical.
Russell U.S. Pat. No. 4,867,902 describes the use of PEG to encapsulate alkali metal superperoxides which release oxygen through the PEG binder. When formed into mats or fabrics, they are described as useful for filters in breathing masks. The encapsulation process involves the sequential steps of melting the PEG, mixing in the powdered chemicals, forming coatings or layers, and allowing the PEG to resolidify at room temperature. See, column 5, lines 22-61.
The use of PEG as a binder has been limited by the AI loading levels permitted by the molten PEG. Experience has shown that agricultural chemicals can be effectively loaded up to only about 55% by weight. Higher loading levels of AI would be useful to provide a more economic use of PEG binders for encapsulation and a smaller volume of encapsulated materials.